Recently, photonic crystals have been drawing attentions as a new optical device. A photonic crystal is an optical functional material having a periodic distribution of refractive index, which provides a band structure with respect to the energy of photons. One of its particular features is that it has an energy region (called the photonic bandgap) that does not allow the propagation of light.
An example of the application fields of the photonic crystal is the optical communication. Recent optical communications use the wavelength division multiplexing (WDM) in place of a conventional method called the time division multiplexing (TDM). Wavelength division multiplexing is a communication method in which plural wavelengths of light, each carrying a different signal, propagate through a single transmission line. This method has drastically increased the amount of information that can be transmitted per unit of time.
Wavelength division multiplexing needs a light source for each of the plural wavelengths. Among the light sources currently used, one type uses semiconductor lasers each having a different oscillation wavelength corresponding to each wavelength, and another type uses a white light source combined with an optical demultiplexer. These methods, however, inevitably increase the size of the device, and are inefficient.
It has been already known that photonic crystals can be used as optical resonators. Since an optical resonator is capable of confining light, an appropriate means for extracting light enables it to be used as a light source. Use of a photonic crystal as a light source will significantly reduce the size of the wavelength division multiplexing optical communication devices.
A study has been conducted on the use of photonic crystals, specifically two-dimensional photonic crystals, as optical resonators (as described in the Japanese Unexamined Patent Publication No. 2001-272555, for example). According to the document, a point defect and a line defect that create a disorder in the periodicity of the photonic crystal are introduced to create an energy level (defect level) caused by the defect within the photonic bandgap. This allows the light to exist only at wavelengths corresponding to the energy of the defect level within the wavelength range corresponding to the photonic bandgap. The line defect functions as a waveguide, and the point defect functions as an optical resonator. When a ray of light having a specific resonance wavelength included in the white light is introduced through the waveguide into the optical resonator, the light resonates at the optical resonator and is emitted from the two-dimensional surface to the outside. Thus, the two-dimensional photonic crystal can be used as a light source emitting the specific wavelength of light. The application of the two-dimensional photonic crystal is not limited to the optical resonator (or light source); its application to wavelength multiplexing/demultiplexing devices is also considered.
For the above-described conventional two-dimensional photonic crystal, it is assumed that the optical resonator uses only a single point defect for each wavelength. However, such a simple structure including only a single point defect allows a portion of the light passing through the waveguide to be reflected at around the waveguide and the optical resonator or pass through the waveguide without entering the optical resonator (i.e. be transmitted through the optical resonator). The light reflected thereby is called the “reflected light” and the light transmitted thereby is called the “transmitted light” hereinafter. The presence of the reflected light and the transmitted light deteriorates the light-extracting efficiency of the optical resonator. Practically, the light-extracting ratio, which is defined as the ratio of the amount of light emitted from the optical resonator to the outside to the amount of the component of light having the resonance wavelength within the white light introduced into the waveguide, could not be higher than approximately 50%.
The present invention has been devised to solve such a problem, and one of its objects is to provide a two-dimensional photonic crystal optical resonator having high light-extracting efficiency. The present invention also intends to demonstrate that the same structure can be applied to optical reflectors.